1. Field of the Invention
The invention concerns a light waveguide, the one end of which comprises a flat entering surface for the light to be coupled into the core of the light waveguide, as well as an optical spectrometer with such a light waveguide.
2. Description of Related Art
An exit slit is necessary in an optical spectrum analyzer at the exit of its optics in the path of light in order to obtain a wavelength selection. The width of the exit slit determines, together with the other parameters of the optics, the wavelength resolution of the spectrum analyzer. For a high wavelength resolution with a size of the spectrum analyzer as small as possible, it is advantageous if the optics of the spectrum analyzer are limited by diffraction, i.e. the resolution is not limited by errors of the optics but by the wave nature of light. In optical communications it is common practice to couple light with a wavelength of about 1.25 μm to about 1.65 μm into a single mode glass fiber light waveguide. In this case, the optimal wavelength resolution is reached for a slit width of about 10 μm. For even smaller slit widths the wavelength resolution does not improve anymore, only the attenuation increases.
In order to produce an input slit for a light waveguide it is known in the art to use a freely supported slit in form of a thin metal foil. A slit with the desired width is created in the metal foil by suitable treatment, for example by means of a high power laser. However, this slit causes undesired polarization dependent loss (PDL). This PDL is the higher the thicker the foil is and the narrower the slit is. In particular, the PDL becomes extremely high if the slit width comes close to the range of the diffraction limitation.
Such a metal aperture can also be adjusted and glued onto the end of the light waveguide. However, adjustment and gluing involves a relatively large expenditure and additionally the reflecting metal aperture causes undesired back reflections.
Finally, the single mode glass fiber itself can be used as input slit, where the core diameter of the single mode glass fiber is about 9 μm (for a mode field diameter of about 10.5 μm and a wavelength of 1.55 μm). However, the exiting slit is then circular, such that already for small changes of the entering light ray relative to the exiting slit perpendicular to the direction of dispersion of the light a large increase of attenuation occurs. With changes in the ambience conditions (for example temperature, ageing) or mechanical strain (for example shock, vibration) there is a high danger that a power indicator shows a wrong value.
In contrast, it is the object of the invention to design a light waveguide of the aforementioned type with an input slit which has a low backreflection and can be produced with relatively low expenditure.